Manufacture of an infra-red detector element, and detection elements so manufactured

ABSTRACT

The manufacture of an infra-red radiation detector element (51). A body of infra-red sensitive material (e.g. cadmium mercury telluride) secured to a substrate (22) is subjected to ion-etching to remove part of the material over the whole thickness of the element (51) at areas (56) between the contact areas of the element electrodes (35 and 36b) so as to define a current path between these electrodes (35 and 36b) which extends through the remaining material and is longer than the distance along a straight line between the electrodes (35 and 36b). This longer current path increases the charge-carrier transit time and resistance between the electrodes (35 and 36b) so that the responsivity of the detector element (51) can be improved while still producing a compact element structure because of the advantages of defining the current path by ion-etching. This etch-removal may involve forming parallel slots (56) which extend from opposite side walls of the element (51) to define a meandering current path, and may additionally be used to form a mutual separation (55) between portions (51 and 52) of the body each of which is to form a separate infra-red detector element of an array formed on the substrate (22).

This is a continuation, of application Ser. No. 059,875, filed July 23,1979 now abandoned.

The invention relates to methods of manufacturing an infra-red detectorelement, particularly but not exclusively of cadmium mercury telluride,and further relates to infra-red detector elements manufactured by suchmethods.

U.S. Pat. No. 4073969 discloses a method of manufacturing a detectorelement for infra-red radiation comprising an active area of infra-redsensitive material between two separate electrodes each on a contactarea of the material; in this method steps are performed to reduce lossin responsivity of the detector element due to recombination of theradiation-generated charge carriers from said active area at one of saidcontact areas.

This loss in responsivity results from a phenomenon called "sweep-out".The responsivity in proportional to the time the radiation-generatedcarriers spend in the infra-red sensitive material. The electrodes atthe contact areas act as regions of intense recombination of theseradiation-generated carriers. The carrier lifetime is a measure of thetime a minority carrier can spend in the infra-red sensitive materialbefore recombining therein with majority carriers. If before thisrecombination occurs in the material, the minority carriers reach thecontact area to which they are travelling, a part of their useful lifein radiation detection is lost and they are said to have been swept outby the electrode. Thus, the electrode is draining away these minoritycarriers faster than they would normally recombine if contacts were notpresent.

The steps disclosed in said United States patent to reduce this loss ofresponsivity consist of shielding part of the detector active area toretain a reduced active area sufficiently spaced from one of the contactareas to permit negligible loss in responsivity due to recombination atthat contact area. However a disadvantage of this technique is that fora given body size the active area is reduced and the shielded partoccupies a significant space which is not sensitive to the radiation;this space occurs over the whole width of the detector element, betweenthe contact area and the reduced active area. Thus, it can be difficultto obtain an array of such detector elements having a maximumradiation-sensitive area with the individual active areas as close aspossible to each other to minimize the non-sensitive areas therebetween.

According to the present invention a method of manufacturing a detectorelement for infra-red radiation, comprising an active area of infra-redsensitive material between two separate electrodes each on a contactarea of the material, in which steps are performed to reduce loss inresponsivity of the detector element due to recombination ofradiation-generated charge carriers from said active area at one of saidcontact areas, is characterized in that said steps comprise securing toa substrate a body of said material from which said detector element isformed, and using ion-etching to remove part of said material frombetween where the contact areas are to be or are formed, saidion-etching being effected over the whole thickness of the body andacross part of its width so as to define a meandering current path inthe active area between the contact areas, which current path extendsthrough the remaining infra-red sensitive material in said active areaand is longer than the distance along a straight line between thecontact areas.

The meandering current path (and hence the element structure) can bevery compact because of the advantages of using ion-etching to removethe material to define the current path. Because of this meanderingcurrent path many of the radiation-generated charge carriers near theone contact area cannot travel in a straight line to said contact areabut must travel along a longer path so increasing their transit time inthe infra-red sensitive material and reducing the loss in detectorresponsivity caused by recombination at the electrode at that contactarea. The longer current path which meanders one or more times increasesnot only the charge-carrier transit time but also the resistance betweenthe electrodes. Because the responsivity is proportional to both thetransit time and the resistance, this increase in resistance furtherincreases the responsivity of the detector element.

Furthermore, in devices in accordance with the invention, the activearea of the detector element can extend right up to the contact areas,and the outside dimensions of the general area which is between thecontact areas and within which the meandering current path is formed inthe active area need not be smaller than that of a detector elementwithout the meandering current path. Thus such detector elementsmanufactured in accordance with the invention can provide aclosely-spaced array of active areas on the substrate.

Ion etching is a known process by which a surface is eroded bybombardment with high energy particles obtained by accelerating ionizedatoms or molecules, at least some of which may be neutralised withelectrons before reaching the surface. Ion energies of a few hundred toseveral thousand eV are generally used.

The Applicants have found that by using such energies, unmasked parts ofbodies of infra-red sensitive material such as, for example, cadmiummercury telluride can be readily etched in a reproducible mannerthroughout their thickness while the effect on the resulting bodysurface need not significantly increase the low frequency (1/f) noise orreduce the detectivity (D*). Steep side-walls can be formed by suchion-etching with at most only small lateral etching occurring below theedge of the etchant mask. Thus, the ion-etching permits very narrowparts of the body to be removed so as to define the longer current pathbetween the contact areas; the width of the narrow parts removed may befor example approximately equal to or less than the thickness of saidbody, and gaps as small as for example 10 microns and less can beobtained in this way. Thus, using ion-etching to form such a longercurrent path has a considerable advantage over the use of chemicaletchant solutions which would etch the body laterally as well asvertically; in that case which is not in accordance with the presentinvention the width of the resulting gaps would be much larger thaneither the thickness of the body or the width of the etchant maskinglayer window. In general, it is desirable to reduce the width of suchgaps because they constitute non-sensitive areas which penetrate theinfra-red sensitive active area of the element between its electrodes.

A meandering current path between the contact areas can be formedcomparatively easily in accordance with the invention when saidion-etching over the whole thickness of the body forms a plurality ofsubstantially parallel slots which extend from opposite side walls ofthe element.

A further advantage of ion-etching is that it may be effected afterproviding a passivating layer at least on the body surface between thecontact areas and/or after providing electrode metallization at least onthe contact areas, because such etching can also remove parts of thepassivating layer and electrode metallization if so desired.

Thus, the ion-etching may be used to remove parts of such a passivatinglayer before removing underlying material over the whole thickness ofthe body. Because such etching can be effected without significantlateral etching of the passivating layer below the mask used, the uppersurface of the active area between the contact areas can remain coveredby the remaining parts of said passivating layer even after forming aquite narrow current path by the material removal.

The ion-etching may remove electrode metallization parts from the bodyand/or the substrate to define the required electrode pattern for thedetector element. Overlying parts of the metallization pattern may beremoved by the ion-etching before removing underlying material over thewhole thickness of the body. This can facilitate manufacture of thedetector element and is especially useful for increasing manufacturingtolerances in aligning the desired ion-etching pattern with the desiredcontact pattern. For the same reasons it is usually preferable for theion-etching to form adjacent at least one contact area a slot whichextends across part of the width of said active area. Thus an elementstructure can be formed in which the part of the active area adjacentthe contact area where sweep-out occurs is reduced and the meanderingcurrent path adjoins this contact area over only part of the width ofsaid active area. In order to facilitate manufacture and retain as muchinfra-red sensitive material as possible between the contact areas, itis preferable for such a slot adjacent the contact area(s) to be etchedat least partially through part of the electrode metallization, therebyremoving a non-sensitive portion of the detector element.

Parts of the electrode metallization may be removed by the ion-etchingso as to divide the metallization into separate electrodes.

The ion-etching step over the whole thickness of the body mayadditionally be used to form a mutual separation between portions of thebody, each of which is to form a separate infra-red detector elementhaving its own contact areas and electrodes. Thus by merely changing thepattern of the masking layer used during the ion-etching and by startingwith an appropriately sized body, a group of detector elements can beformed by the same processing steps.

In order to illustrate the realisation of these and other features inaccordance with the invention and their advantages, embodiments of theinvention will now be described, by way of example, with reference tothe accompanying diagrammatic drawings, in which:

FIG. 1 is a cross-sectional view of a wafer of cadmium mercury telluridemounted on a polishing block at an early stage in the manufacture of aninfra-red detector element by a method in accordance with the invention;

FIG. 2 is a cross-sectional view of the wafer mounted on anotherpolishing block at a subsequent step in the method;

FIG. 3 is a cross-sectional view of the wafer after thinning on saidother polishing block;

FIG. 4 is a cross-sectional view of the thinned wafer on said otherpolishing block at a subsequent ion-beam etching step for forming stripportions from the wafer;

FIG. 5 is a plan view of the thinned wafer on said other polishing blockafter the ion-beam etching step, the cross-section of FIG. 4 being takenalong the line IV--IV of FIG. 5;

FIG. 6 is a cross-sectional view taken along the same line as FIG. 4after a thinning and rounding step;

FIG. 7 is a cross-sectional view of parts of three of the strip portionsof the wafer after a subsequent anodising treatment;

FIG. 8 is a cross-sectional view along the line VIII--VIII of FIG. 5illustrating the division of a strip portion of the wafer into separatebodies during a further ion-beam etching step;

FIG. 9 is a plan view of one such body mounted on a detector substratein a subsequent step of the manufacture;

FIG. 10 is a plan view of the arrangement of FIG. 9 after providing afirst masking layer;

FIG. 11 is a cross-sectional view taken on the line XI--XI of FIG. 10after forming a mesa at the body surface by ion-beam etching;

FIG. 12 is a cross-sectional view of the arrangement shown in FIG. 11after a metal deposition step;

FIG. 13 is a plan view of the arrangement of FIG. 12 after removal ofthe first masking layer;

FIG. 14 is a plan view of the arrangement of FIG. 13 after providing asecond masking layer for determining the desired pattern of detectorelements and their electrodes;

FIG. 15 is a cross-sectional view of the arrangement of FIG. 14 duringan ion-beam etching step to form said desired pattern;

FIG. 16 is an enlarged plan view of part of the resulting detectorelement array which is in accordance with the invention;

FIG. 17 is a cross-sectional view taken on the line XVII--XVII of FIG.16;

FIG. 18 is a modification of FIG. 16 showing in plan view a detectorelement in accordance with the invention and manufactured by a slightlydifferent method also in accordance with the invention,

and

FIG. 19 is a modification of FIG. 18 showing in plan view a detectorelement in accordance with the invention and manufactured by a slightlydifferent method also in accordance with the invention.

It should be noted that the Figures in the accompanying drawings are notdrawn to scale; the relative dimensions and proportions of some parts ofthe Figures have been greatly exaggerated or reduced for the sake ofclarity. In particular, the thickness of the various layers in relationto their lateral extent is much smaller than may otherwise be apparentfrom the drawings. The same reference numerals as used in one Figure aregenerally used to refer to the same or similar parts in other Figuresand embodiments.

In the method now to be described with reference to FIGS. 1 to 17, thestarting material is a crystal wafer 1 of cadmium mercury telluride,Cd_(1-x) H_(gx) Te where 0<x<1. The material is sensitive to infra-redradiation, and the atomic ratio of cadmium to mercury may be such as toproduce a cut-off wavelength for the material of for exampleapproximately 12 microns. The dimensions of the starting wafer are notcritical but should be sufficient to provide the large number ofdetector elements to be manufactured therefrom, for example over athousand elements; the wafer may be circular with a diameter of forexample 10 m.m.; its thickness may be for example 0.5 m.m.

The wafer 1 is mounted on a polishing block 2, for example by a layer ofwax 3. The thickness of the wafer 1 projecting above shoulders of theblock 2 is then lapped away and its exposed major surface is polished inknown manner. The resulting wafer thickness may now be for example 200microns. Preferably the final polishing stage involves a chemicaletching treatment to remove surface damage. The polished surfaces andthe side of the wafer 1 are now provided with a passivating layer 4, forexample by anodic oxidation in known manner. The resulting arrangementis illustrated in FIG. 1.

The wafer 1 is now removed from the block 2 and adhered via its anodizedmajor surface to another polishing block 5, for example with a layer ofwax 7. Although the passivating layer 4 is shown in FIGS. 1 and 2, it isomitted in subsequent Figures for the sake of convenience. The thicknessof the wafer 1 projecting above shoulders of the block 5 is then lappedaway and its exposed major surface is polished in known manner. Theresulting wafer thickness may now be for example 15 microns. Theresulting arrangement is illustrated in FIG. 3.

A layer of photoresist is then provided on the thinned wafer 1 and isselectively exposed and developed to provide a photoresist masking layer10. The pattern of the layer 10 corresponds to the pattern ofinterconnected strip portions 9 illustrated in FIG. 5. The photoresistmay be for example that commercially available under the trade nameShipley resist AZ 1350H of Shipley Chemicals Limited. An etchingtreatment is then effected using the layer 10 as an etchant mask so asto form a plurality of slots 8 through the thickness of the wafer 1. Asshown in FIGS. 4 and 5, the slots 8 define substantially parallel stripportions 9 of the wafer which are interconnected by an orthogonal strip11 extending across a diameter of the wafer.

As illustrated in FIG. 4, this etching treatment may be performed by abeam 12 of, for example, argon ions. Such etching is effected in avacuum chamber with the wafer and block arrangement 1, 5 mounted on atarget holder; the target holder may be for example water-cooled androtatable during the etching treatment. The pressure in the chamber iskept sufficiently low that scattering of the ions is minimal and thesurface to be etched is bombarded by the ions at a predetermined angle.The incident ion-beam is obtained from an ion source which may bemounted, for example, at the top of the chamber. The Applicants haveused a chamber with a saddle-field ion source commercially availablefrom Iontech Limited of Teddington, U.K. Using this ion source, theetching area at a target distance of 5 cm. is found to be 2 sq.cm., andthe argon ion current can be up to 45 micro Amps plus an approximatelyequivalent dose of neutral argon atoms using a voltage of 5 kV with asource current of 1 mA and an argon pressure of 5×10⁻ 4 torr.

The etch rate depends on the beam current, the incident angle of thebeam, the energy of the beam and the nature of the target material. Theincident angle can be varied by tilting the target holder. For use atdifferent stages of this method of manufacture, the target holder may beat a distance of, for example, 4 cm. from the ion source with anincident beam which may be perpendicular to the surface or inclined atan angle of up to, for example, 45°; under these conditions theApplicants have etched cadmium mercury telluride at a rate ofapproximately 4 microns an hour. The etch rate of the Shipley resist wasfound to be between 0.1 and 0.3 times that of the cadmium mercurytelluride. Therefore approximately 4 hours are needed to etch the slots8 through the 15 micron thickness of the wafer 1. A typical thicknessfor the layer 10 is 5 to 7.5 microns. By ion-beam etching in this mannerthe Applicants have found that steep side walls having a slope of forexample 75° can be achieved. In this way narrow slots 8 can be formed inthe wafer 1 so that a large proportion of the wafer can be used toprovide the detector elements. The remaining strip portions 9 may be forexample 200 microns wide, and the slots 8 may be for example 10 micronswide etched through 10 micron wide windows in the masking layer 10.Although FIG. 5 shows only fourteen such strip portions 9 across thediameter of the wafer it should be understood that there are actuallymany more present.

In the next stage of manufacture, the part of the photoresist layer 10remaining on the strip portions 9 is removed, after which the thicknessof the strip portions 9 is reduced to for example 10 microns whilerounding their exposed longitudinal upper edges. This thicknessreduction and rounding may be effected by polishing and etching in amanner such as that described in U.S. Pat. No. 4,037,311. FIG. 6 showsin cross-section the strip portions 9 after the etching process; due tothe distortion of the relative dimensions of the drawing the rounding ofthe longitudinal edges is not apparent in this Figure, but it isillustrated in the close-up view of FIG. 7. It should also be noted thathe wax layer parts exposed by the slots 8 are removed during theetching of the slots 8 and during the subsequent thinning and roundingtreatment.

The exposed upper surface of the strip portions 9 and their side wallsare now provided with a passivating layer 14, for example by anodicallyoxidizing the cadmium mercury telluride surface in known manner. Thestrip portion 11 serves to interconnect the strip portions 9 during suchan anodizing treatment. Both the passivating layers 4 and 14 areillustrated in the close-up cross-sectional view of one such stripportion 9 shown in FIG. 7.

A further layer of photoresist is then provided and is selectivelyexposed and developed to form a masking layer 16 having a pattern fordividing the strip portions 9 along their length into a plurality ofseparate bodies 21 of infra-red sensitive material. This is effected byetching through the thickness of the portions 9 so as to form slotswhich extend perpendicular to the slots 8. This etching step also may beeffected in a manner similar to that described with reference to FIG. 4by using a beam 17 of argon ions. This step is illustrated in FIG. 8.The length of each body 21 formed from the strip portions 9 in thisembodiment is sufficient to provide a linear array of two detectorelements as will now be described with reference to FIGS. 9 to 17. Thebodies 21 may thus have for example a length of 125 microns, a width of200 microns and a thickness of 10 microns.

A body 21 is removed from the polishing block 5 and is mounted on anarea of a surface of an insulating substrate 22. The surface of the body21 passivated by the layer 4 is secured to the surface of the substrate22, for example by a thin adhesive layer 23 which is illustrated in thecross-sectional views of FIGS. 11, 12 and 15. The substrate 22 may be ofoptically-polished sapphire. The body 21 is shaded in FIG. 9 for thesake of clarity.

Next a layer of photoresist is provided over the surface of thesubstrate 22 and body 21, and is selectively exposed and developed toform a first photoresist masking layer 24 on a part of the upper surfaceof both the body 21 and the substrate 22. The layer 24 consists of astripe of photoresist which is shown shaded in FIG. 10. The stripe 24extends locally across the body 21 in a direction substantiallyperpendicular to those along which the body 21 will subsequently bedivided into the desired linear array. The stripe 24 is present on thepassivating layer 14 on the body surface where the underlying maskedarea defines the passivated active area of the detector element; in thisactive area the infra-red radiation is to be sensed. The stripe 24 maybe for example 50 microns wide. The areas not masked by the stripe 24are subsequently provided with a metallization pattern as will bedescribed hereinafter. The stripe 24 determines a separation in themetallization pattern on the body 21 and extends onto the substrate 22to also separate parts of the metallization pattern where subsequentlyformed on the substrate 22.

Before depositing metal for said metallization pattern, a mesa is formedat the body surface by ion-beam etching. This is illustrated in FIG. 11.While using the stripe 24 as an etchant mask the exposed part of thepassivating layer 14 and an underlying portion of the infra-redsensitive material is removed by bombardment with a beam 27 of forexample argon ions. The etching conditions may be similar to thosedescribed previously in connection with FIGS. 4 and 8. However thebombardment is effected for a shorter time so that the etching occursthrough only part of the thickness of the body 21. As a result, there isformed a mesa 31 upstanding on the remainder of the body 21; this mesa31 of infra-red sensitive material is topped by the remainder of themasking stripe 24, and the remainder of the passivating layer 14 ispresent between the top of the mesa 31 and the stripe 24. The brokenline 26 in FIG. 11 indicates the original passivated surface of the body21. The ion bombardment also etches to a lesser extent the photoresiststripe 24 and the exposed surface of the substrate 22, but this is notindicated in the drawing for the sake of simplicity.

The Applicants have found that the etch rate of anodic oxide which formsthe layer 14 is approximately 1.3 times that of the underlying cadmiummercury telluride, and that there does not occur any significant removalof the layer 14 under the edge of the masking layer 24 during theion-beam etching; this is important since it is desirable for the layer14 to adjoin at its edges the subsequently provided metal electrodes.Similarly no significant etching occurs of the layer 4 under the edge ofthe body 21.

The Applicants have found that ion-beam etching permits the mesa 31 tobe formed in a reproducible manner with a well-defined shape by uniformremoval of cadmium mercury telluride from the unmasked surface parts ofthe body 21. The cadmium mercury telluride is etched to a uniform depthof at least 0.5 micron and preferably much deeper, for example 2 or 3microns; the resulting structure permits a significant proportion of thecurrent occurring between the detector electrodes in operation of thefinal detector element to flow across the bulk of the mesa 31 betweenits side-walls rather than adjacent its top surface. The ion-beametching permits the side-walls of the mesa 31 to be comparatively steep,for example with a slope of 75°. The factors that control the slopeinclude the angle of the beam 27, the shape and etching of the resist 24and any redeposition of the ejected target atoms.

After forming the mesa 31, metal is deposited to form a layer 33 on thephotoresist stripe 24, on parts of the body 21 not masked by the stripe24, and on the area of the surface of the substrate 22 around the body21. This metal layer 33 is thus deposited on both the side-walls of themesa 31 and on the surface of the remainder of the body 21. Theresulting structure is illustrated in FIG. 12.

Preferably the metal is deposited by evaporation because chromium isdifficult to sputter, and evaporation is a relatively cool, low energyprocess which does not significantly damage the cadmium mercurytelluride. The Applicants have found that metal deposited in this waycan have good adhesion to the topographically rough surfaces produced bythe ion-beam etching on both the cadmium mercury telluride body 21 andthe sapphire substrate 22. However it is of course possible to depositthe metal in other ways, for example by sputtering. The Applicants havefound it is advantageous for the metal to consist of a first layer ofchromium deposited in contact with the infra-red sensitive material(because of the strong adhesion of chromium to both cadmium mercurytelluride and sapphire) and a thicker second layer of gold deposited onthe chromium layer to reduce the electrical resistance of the metalfilm; chromium has both a thermal expansion coefficient and a workfunction compatible to that of cadmium mercury telluride and does notform an amalgam with either mercury or gold at temperatures belowapproximately 150° C. Such an evaporated gold-chromium layer 33 can havea particularly strong adhesion to both the body 21 and the substrate 22.The chromium may be for example 0.05 micron thick, and the gold may befor example 0.5 micron thick.

The masking layer 24 is then removed to lift away the metal thereon andto leave the remainder of the layer 33 as a metallization pattern 35, 36on both the body 21 and the substrate 22, as illustrated in FIG. 13.Since the layer 24 is of photoresist it can be removed by immersing inacetone and possibly using agitation to aid the removal in known manner.The remaining metallization pattern consists of two separate parts 35and 36, each of which extends on the side-walls of the mesa 31 and willbe further processed subsequently to form separate detector electrodescontacting these side-walls of the detector elements.

This further processing involves masking and etching the body 21 and themetallization pattern 35, 36 to divide them into a desired pattern ofdetector elements and their electrodes. For this purpose a secondmasking layer 44 is provided on most of the metallization pattern 35, 36and most of the body 21 where not covered by the pattern 35, 36. Thislayer 44 may also be formed of photoresist such as for example Shipleyresist AZ 1350H and has a plurality of windows 45, 46 and 48 which areformed by selective exposure and development of the photoresist.

As shown in FIG. 14 the stripe-shaped window 45 extends in a directionperpendicular to that along which the earlier stipe 24 extended. Thiswindow 45 defines where the body 21 and the metallization part 36 willeach be divided into two parts, one for each of the two detectorelements to be formed. The width of the window 45 may be for example 10microns. The outer two windows 48 define a side-wall of each of theelements at opposite ends of the body 21.

The six substantially parallel windows 46 extend perpendicularly fromthe windows 45 and 48 so as to form an interdigitated window patternover the parts of the body 21 where the infra-red sensitive active areasof each detector element are to be formed. These windows 46 define thoseparts of the material which are to be removed by the etching over thewhole thickness of the body 21 so as to form interdigitated slots 56which (as shown in FIG. 16) extend from opposite side walls of eachresulting detector element across a major part of the width of eachelement. As a result of these interdigitated slots each element has ameandering current path formed between its electrodes as will bedescribed hereinafter. The widths of the windows 46 in the masking layer44 may be for example 7.5 microns.

The parts of the body 21 and the metallization pattern 35, 36 at thesewindows 45, 46 and 48 are now removed from the substrate 22. Thisremoval is effected by etching with a beam 47 of argon ions in a mannersimilar to that described with reference to FIG. 8. The etching iseffected throughout the thickness of the body 21 and throughout thethickness of the layer pattern 35, 36 while using the masking layer 44as an etchant mask.

The steep side-walls produced by ion-beam etching of the cadmium mercurytelluride and the only very small lateral etching which occurs permitboth a close separation 55 between the resulting elements 51 and 52 andthe formation of very narrow slots 56 which extend from opposite sidewalls of the elements to form the meandering current path. Theseparation 55 may be for example 10 microns, and the width of the slots56 may be for example 7.5 microns. This is a considerable advantage overthe use of chemically reactive etchant solutions for such etching.

The argon ions etch away the exposed parts of the body 21 including thecorresponding parts of its anodic surface layers 14 and 4 withoutsignificant lateral etching of these layers 14 and 4 under the edges ofthe masking layer 44 and the resulting elements respectively. The argonions also etch away the exposed metallization in the same etching step.The etching is continued for a sufficiently long time to etch throughthe parts of the body 21 where covered by exposed parts of themetallization pattern 35, 36. This etching of body parts under exposedmetallization parts occurs at the slots 45 and at the outer slots 46where the material is removed from adjacent the contact areas of theelectrode metallization 35 and 36.

After removing the second masking layer 44, the resulting detectorarrangement is as shown in FIGS. 16 and 17. The group of two detectorelements 51 and 52 so formed have a common electrode 35 on one side andon the opposite side individual electrodes 36a and 36b formed from part36 of the metallization pattern 35, 36. Each of these detector elementscomprises an active area in the form of a mesa 31 of the infra-redsensitive material with the separate, sunken metal electrodes 35 and forexample 36a on the opposite side-walls of the mesa 31, as illustratedfor detector element 52 in FIG. 17. Although such mesa contacting is notessential in a method in accordance with the present invention, it canlead to significant performance benefits for the detector; thistechnique is described and claimed in one of our co-pending PatentApplications PHB 32631 filed on the same day as the present Applicationand the contents of the Specification of which are hereby incorporatedby reference into the present Specification.

Because of both the separation 55 between the elements 51 and 52 and theslots 56 which extend into them are narrow, the area which they occupyis small which is important as this is a non-sensitive area of thedetector. Because of the slots 56, the current path meanders between theelectrodes of each element 51 and 52 and adjoins both the electrodecontact areas of each element over only part of the width of the wholeactive area of the element. This current path is therefore longer thanthe distance along a straight line between the areas contacted by theelectrodes of that element. This increases the charge-carrier transittime and the resistance between the electrodes and so can improve theresponsivity of each of the detector elements while still producing acompact detector structure. By including slots 56 in this manner, theApplicants have increased by a factor of between 3 and 4 theresponsivity of a detector element having a sensitive area of 50 micronsby 50 microns.

The fabrication process described with reference to FIGS. 9 to 17requires only two masking steps. The first mask 24 determines ametallization pattern and its alignment is not critical. The second mask44 determines the desired pattern of elements and their electrodes whichare formed from the body 21 and the metallization pattern respectively.Such a process is described and claimed in our PHB 32630 PatentApplication which is filed on the same day as the present Applicationand to which reference is invited; it has an advantage in not requiringa critical alignment of separate masking steps, one for dividing thebody 21 into elements and another for defining the electrode pattern, inspite of the very small spacing between adjacent elements of the array.

External connections can be made to the elements of the array by bondingwires to the parts of the electrodes 35, 36a and 36b where they arepresent directly on the substrate 22 so as not to damage the infra-redsensitive material of the elements 51 and 52.

Many modifications are possible within the scope of the presentinvention.

By forming a body 21 of different size and/or by using a differentpattern for the first masking layer 24 and/or the second masking layer44, different groups of detector elements can be formed on the substrate22. Such groups may be for example linear arrays of more than twoelements, back-to-back linear arrays which together form atwo-dimensional area array, and for example staggered arrays of detectorelements.

Instead of dividing the strip portions 9 shown in FIG. 8 into bodies 21or sufficient size to form a plurality of detector elements, the stripportions 9 may be divided into bodies 21 for forming a single detectorelement. After securing such a body to an insulating substrate 22, itmay be provided with a first masking layer such as the stripe 24 in FIG.10, ion-beam etched, and then provided with a metallization patternsimilar to the pattern 35, 36 of FIG. 13 by metal deposition and maskremoval. In this case, the metallization pattern thus formed may beidentical to the final electrode pattern because it does not need to beformed into separate electrodes for separate detector elements. Thebody, substrate and metallization pattern are then masked with a secondmasking layer which need only have windows determining where parts ofthe body 21 are to be removed by ion-beam etching from the substrate 22to define a meandering current path between the element electrodes. FIG.18 is an example of a detector element 71 manufactured in this mannerand having electrodes 35 and 36. In this embodiment the slots 76defining the current path could have had the same form as the slots 56of each element shown in FIG. 16. However FIG. 18 illustrates a modifiedform of the slots which is applicable also to the FIGS. 16 and 17embodiment; as shown in FIG. 18, there are only two slots 76 and each ofthem extends into the element body from an opposite side wall and thenturns through 90° to extend parallel to the length of the element 71. Itshould be evident that many other slot structures are possible forforming a meandering current path between the electrodes, and that thenumber of meanders which the current path makes can be increased byincreasing the number of slots 56 or 76 which extend from the side wallsof the detector elements.

FIG. 19 illustrates a modification of the FIG. 18 structure, in whichthe L-shape slots 76 formed by the ion-etching extend into the elementbody to divide the metallization pattern into end electrodes 85 and 86and intermediate sunken electrodes 80 which extend across intermediateparts of the current path. The same method steps in accordance with theinvention are used, but with elongation and displacement of the L-shapewindows in the mask for the ion-etching definition of the meanderingcurrent path. As before, the increased resistance reduces theresponsivity loss.

The ion-beam etching illustrated in FIG. 15 forms exposed surfaces ofthe detector elements at both the slots 56 and the separation 55. Thesesurfaces can be passivated by subsequently forming a passivating layerin known manner, although it appears to the Applicants that the ion-beametched surfaces already have some intrinsic passivation, perhaps as aresult of implantation of the inert ions at the surface. Instead offorming a passivating layer 14 on the upper surface of the detectorelements before metallization and element definition, the sensitiveactive areas of the elements and their sides can be passivated by asubsequent treatment.

The Applicants have found that ion-beam etching (especially with atleast part of the ion beam neutralized with electrons) has proved to bea particularly reproducible etching process for infra-red sensitivematerials such as cadmium mercury telluride, while avoiding seriousdamage to the material. However instead of ion-beam etching, otherequivalent forms of ion-etching may be used, for example so-called"magnetron sputtering" in which the ion flux used for sputter-etching isconcentrated by a magnetic field. Magnetron sputtering is described infor example the article entitled "Equipment for sputtering" by A. J.Aronson, Solid State Technology, December 1978, pages 66 to 72, althoughthis article is primarily concerned with sputter-deposition rather thansputter-etching. Other ion-etching processes are described in thearticles "An Investigation of Ion-Etching" by H. Dimigen et al., PhilipsTechnical Review, Vol. 35, No. 7/8, pages 199 to 208, and "Introductionto Ion and Plasma Etching" by S. Somekh, Journal of Vacuum ScienceTechnology, Vol. 13, No. 5, pages 1003 to 1007.

Instead of forming the detector elements of cadmium mercury telluride,other infra-red sensitive materials may be employed, for example otherternary intermetallic chalcogenides such as for example lead tintelluride or other monocrystalline semiconductors such as for examplelead sulphide or indium antimonide.

In the embodiments described hereinbefore the methods comprise theapplication of ohmic contact electrodes to element bodies having auniform material composition and for use in detectors of which theoperation is based on intrinsic photoconductivity. However also withinthe scope of the present invention is the manufacture of detectorelements each of which has a p-n junction located on the sensitive mesaarea of the element body; in this case the element has electrodes whichextend on the side-walls of the mesa and make ohmic contact to thep-type and n-type regions respectively of the body.

It will also be evident that other metals than gold and chromium may beused to form the electrodes, for example aluminium or silver, and thatthe detector substrate may be of material other than sapphire. Thus, forexample the insulating substrate 22 may be of for example alumina,silicon or beryllia.

What is claimed is:
 1. A method of manufacturing a detector element forinfra-red radiation, which element comprises infra-red sensitivematerial having an active area and two parallel electrodes each disposedon a contact area of the infra-red sensitive material so that the activearea lies between the electrodes, comprising the steps of:(a) securing abody of infra-red sensitive material to a substrate; (b) thereafterproviding a masking layer on the body, the masking layer having elongatewindows between areas on the body where the two contact areas are to beor are formed, the elongate windows exposing stripe-shaped parts of thebody which extend at least transverse to the direction between the twocontact areas; and (c) ion-etching the material through the elongatewindows in the masking layer, over the whole thickness of the body, toform substantially parallel slots in the active area which extend fromopposite side-walls of the detector element across part of the width ofthe active area to define a meandering current path in the active areabetween the two contact areas, which current path extends through theinfra-red sensitive material remaining in said active area and is longerthan the distance along a straight line between the two contact areas.2. A method as claimed in claim 1, further comprising the steps of firstproviding a passivating layer at least on the surface of the bodybetween the contact areas, then providing the masking layer and thenion-etching to remove parts of the passivating layer before removing theunderlying material over the whole thickness of the body.
 3. A method asclaimed in claim 2 wherein the width of the slots is at most equal tothe thickness of the body.
 4. An infra-red detector manufactured by themethod claimed in claim
 3. 5. A method as claimed in claim 2 wherein thewidth of the slots is at most 10 microns.
 6. A method as claimed inclaim 5 wherein the masking layer comprises at least one further windowwhich extends parallel to the direction between the two contact areasand from which the elongate windows extend transversely, and wherein theion-etching forms at the further window a separation between portions ofthe body so that each portion forms a separate infra-red detectorelement having a meandering current path between its contact areas andelectrodes.
 7. A method as claimed in claim 6 wherein the step ofion-etching divides the electrode metallization to form both anelectrode at one end of the meandering current path and an intermediateelectrode on the current path.
 8. An infra-red detector elementmanufactured by the method claimed in claim
 7. 9. An infra-red detectormanufactured by the method claimed in claim
 6. 10. A method as claimedin claim 5 wherein the step of ion-etching over the whole thickness ofthe body includes forming a separation between parts of the body so thateach portion forms a separate infra-red detector element having its owncontact areas and electrodes.
 11. A method as claimed in claim 5 whereinthe step of ion-etching divides the electrode metallization to form bothan electrode at one end of the meandering current path and anintermediate electrode on the current path.
 12. An infra-red detectorelement manufactured by the method claimed in claim
 11. 13. An infra-reddetector manufactured by the method claimed in claim
 5. 14. A method asclaimed in claim 1 or claim 2, further comprising the step of providingelectrode metallization, at least on the contact areas of the body whileleaving at least part of the active area between the contact areas freeof metallization, and wherein the ion-etching is effected afterproviding the electrode metallization so that part of the metallizationis removed by the ion-etching.
 15. A method as claimed in claim 14wherein the step of ion-etching divides the electrode metallization toform an electrode at one end of the meandering current path and anintermediate electrode on the current path.
 16. An infra-red detectormanufactured by the method claimed in claim
 15. 17. A method as claimedin claim 14, wherein the electrode metallization extends from the bodyonto the substrate, and the masking layer has windows which extend fromthe body to the substrate to define a desired electrode pattern on thesubstrate and further comprising the step of removing parts of themetallization from the substrate through said windows during the ionetching step.
 18. A method as claimed in claim 14 wherein the width ofthe slots is at most equal to the thickness of the body.
 19. Aninfra-red detector manufactured by the method claimed in claim
 18. 20. Amethod as claimed in claim 14 wherein the width of the slots is at most10 microns.
 21. An infra-red detector manufactured by the method claimedin claim
 20. 22. A method as claimed in claim 14 wherein the step ofion-etching over the whole thickness of the body includes forming a slotwhich extends across part of the width of the active area adjacent atleast one contact area.
 23. A method as claimed in claim 22 wherein thestep of ion-etching over the whole thickness of the body includesforming a separation between parts of the body so that each portionforms a separate infra-red detector element having its own contact areasand electrodes.
 24. A method as claimed in claim 22 wherein the step ofion-etching divides the electrode metallization to form both anelectrode at one end of the meandering current path and an intermediateelectrode on the current path.
 25. An infra-red detector manufactured bythe method claimed in claim
 24. 26. An infra-red detector manufacturedby the method claimed in claim
 22. 27. A method as claimed in claim 14wherein said masking layer comprises at least one further window whichextends parallel to the direction between the two contact areas and fromwhich the elongate windows extend transversely, and wherein theion-etching forms at the further window a separation between portions ofthe body so that each portion forms a separate infra-red detectorelement having a meandering current path between its contact areas andelectrodes.
 28. An infra-red detector manufactured by the method claimedin claim
 27. 29. A method as claimed in claim 14 wherein the step ofion-etching over the whole thickness of the body includes forming aseparation between parts of the body so that each portion forms aseparate infra-red detector element having its own contact areas andelectrodes.
 30. An infra-red detector manufactured by the method claimedin claim
 14. 31. A method as claimed in claim 1 or claim 2 wherein thestep of ion-etching over the whole thickness of the body includesforming a slot which extends across part of the width of the active areaadjacent at least one contact area.
 32. A method as claimed in claim 31wherein the width of the slots is at most equal to the thickness of thebody.
 33. A method as claimed in claim 32 wherein the width of the slotsis at most 10 microns.
 34. A method as claimed in claim 33 wherein themasking layer comprises at least one further window which extendsparallel to the direction between the two contact areas and from whichthe elongate windows extend transversely, and wherein the ion-etchingforms at the further window a separation between portions of the body sothat each portion forms a separate infra-red detector element having ameandering current path between its contact areas and electrodes.
 35. Aninfra-red detector manufactured by the method claimed in claim
 34. 36. Amethod as claimed in claim 33 wherein the step of ion-etching over thewhole thickness of the body includes forming a separation between partsof the body so that each portion forms a separate infra-red detectorelement having its own contact areas and electrodes.
 37. An infra-reddetector manufactured by the method claimed in claim
 33. 38. A method asclaimed in claim 32 wherein the masking layer comprises at least onefurther window which extends parallel to the direction between the twocontact areas and from which the elongate windows extend transversely,and wherein the ion-etching forms at the further window a separationbetween portions of the body so that each portion forms a separateinfra-red detector element having a meandering current path between itscontact areas and electrodes.
 39. An infra-red detector manufactured bythe method claimed in claim
 38. 40. A method as claimed in claim 32wherein the step of ion-etching over the whole thickness of the bodyincludes forming a separation between parts of the body so that eachportion forms a separate infra-red detector element having its owncontact areas and electrodes.
 41. An infra-red detector manufactured bythe method claimed in claim
 32. 42. A method as claimed in claim 31wherein the width of the slots is at most 10 microns.
 43. A method asclaimed in claim 42 wherein the masking layer comprises at least onefurther window which extends parallel to the direction between the twocontact areas and from which the elongate windows extend transversely,and wherein the ion-etching forms at the further window a separationbetween portions of the body so that each portion forms a separateinfra-red detector element having a meandering current path between itscontact areas and electrodes.
 44. An infra-red detector manufactured bythe method claimed in claim
 43. 45. A method as claimed in claim 42wherein the step of ion-etching over the whole thickness of the bodyincludes forming a separation between parts of the body so that eachportion forms a separate infra-red detector element having its owncontact areas and electrodes.
 46. An infra-red detector manufactured bythe method claimed in claim
 42. 47. A method as claimed in claim 31,further comprising the step of ion-etching through an L-shaped window inthe masking layer adjacent each of said two contact areas to form slots,each slot having a first portion which is adjacent said contact area andextends from a side-wall of the detector element and a second portionwhich extends transverse to the first portion in a direction away fromthe contact area.
 48. A method as claimed in claim 47, furthercomprising the step of providing electrode metallization at least on thecontact areas of the body while leaving at least part of the active areabetween the contact areas free of the metallization, wherein at leastone of the L-shaped windows exposes a part of the metallization andwherein said ion-etching is effected after providing the metallizationso that the ion-etching removes said part of the metallization togetherwith the underlying part of the body.
 49. A method as claimed in claim48 wherein the step of ion-etching divides the electrode metallizationto form both an electrode at one end of the meandering current path andan intermediate electrode on the current path.
 50. An infra-red detectormanufactured by the method claimed in claim
 49. 51. A method as claimedin claim 31 wherein the masking layer comprises at least one furtherwindow which extends parallel to the direction between the two contactareas and from which the elongate windows extend transversely, andwherein the ion-etching forms at the further window a separation betweenportions of the body so that each portion forms a separate infra-reddetector element having a meandering current path between its contactareas and electrodes.
 52. An infra-red detector manufactured by themethod claimed in claim
 51. 53. A method as claimed in claim 31 whereinthe step of ion-etching over the whole thickness of the body includesforming a separation between parts of the body so that each portionforms a separate infra-red detector element having its own contact areasand electrodes.
 54. An infra-red detector manufactured by the methodclaimed in claim
 31. 55. A method as claimed in claim 1 or claim 2wherein the masking layer comprises at least one further window whichextends parallel to the direction between the two contact areas and fromwhich the elongate windows extend transversely, and wherein theion-etching forms a separation between portions of the body at thefurther window so that each portion forms a separate infra-red detectorelement having a meandering current path between its contact areas andelectrodes.
 56. An infra-red detector manufactured by the method claimedin claim
 55. 57. A method as claimed in claim 1 or claim 2 wherein theion-etching is effected using an ion-beam.
 58. An infra-red detectormanufactured by the method claimed in claim
 57. 59. An infra-reddetector element manufactured by the method claimed in claim 1 or claim2.
 60. A method as claimed in claim 1 wherein the width of the slots isat most equal to the thickness of the body.
 61. A method as claimed inclaim 60 wherein the width of the slots is at most 10 microns.
 62. Amethod as claimed in claim 61 wherein the masking layer comprises atleast one further window which extends parallel to the direction betweenthe two contact areas and from which the elongate windows extendtransversely, and wherein the ion-etching forms at the further window aseparation between portions of the body so that each portion form aseparate infra-red detector element having a meandering current pathbetween its contact areas and electrodes.
 63. An infra-red detectormanufactured by the method claimed in claim
 62. 64. A method as claimedin claim 61 wherein the step of ion-etching over the whole thickness ofthe body includes forming a separation between parts of the body so thateach portion forms a separate infra-red detector element having its owncontact areas and electrodes.
 65. A method as claimed in claim 64wherein the step of ion-etching divides the electrode metallization toform both an electrode at one end of the meandering current path and anintermediate electrode on the current path.
 66. An infra-red detectorelement manufactured by the method claimed in claim
 65. 67. An infra-reddetector manufactured by the method claimed in claim
 64. 68. A method asclaimed in claim 60 wherein the masking layer comprises at least onefurther window which extends parallel to the direction between the twocontact areas and from which the elongate windows extend transversely,and wherein the ion-etching forms at the further window a separationbetween portions of the body so that each portion forms a separateinfra-red detector element having a meandering current path between itscontact areas and electrodes.
 69. A method as claimed in claim 68wherein the step of ion-etching divides the electrode metallization toform both an electrode at one end of the meandering current path and anintermediate electrode on the current path.
 70. A infra-red detectormanufactured by the method claimed in claim
 68. 71. A method as claimedin claim 60 wherein the step of ion-etching over the whole thickness ofthe body includes forming a separation between parts of the body so thateach portion forms a separate infra-red detector element having its owncontact areas and electrodes.
 72. An infra-red detector manufactured bythe method claimed in claim
 60. 73. A method as claimed in claim 60wherein the step of ion-etching divides the electrode metallization toform both an electrode at one end of the meandering current path and anintermediate electrode on the current path.
 74. A method as claimed inclaim 1 wherein the width of the slots is at most 10 microns.
 75. Aninfra-red detector manufactured by the method claimed in claim 74.